Infrared Spectroscopy of Diamondoid Molecules: New Insights into the Presence of Nanodiamonds in the Interstellar Medium

Abstract

Although they are relatively different in band shape, infrared featuresaround 3.4-3.5 ÎŒm in the emission spectra of HD 97048 and Elias1 and in the absorption spectra of various dense clouds have bothbeen attributed to diamondoid molecules/particles. This assignmentis based mainly on infrared spectra of hydrogenated diamond thinfilms and of diamond nanocrystals of known average size. Here wepresent an analysis of the astrophysical implications of recentlyreported solid-state 2.5-12.5 ÎŒm spectra of individual diamondoidmolecules, up to the size of hexamantane (C 26 H 30 ). These spectraprovide the first experimental measurements of the infrared frequenciesof this class of molecules. In addition, laboratory gas-phase infraredemission spectra of the three smallest members of the diamondoidfamily are reported, as well as theoretical spectra for some largerspecies. The present data set allows us to relate spectral signaturesto the molecular size and structure. The spectra of tetrahedral diamondoidsare found to be qualitatively different from those of lower symmetryspecies, which possibly explains the differences between the astrophysicalemission and absorption spectra. Interestingly, the 3.53 ÎŒm bandis clearly observed in the spectra of these small molecular diamondoids,whereas previous studies on nanodiamond particles found this bandonly for species larger than ##IMG## [http://ej.iop.org/icons/Entities/approx.gif]{approx} 50 nm. Our results support the assignment of the 3.43 and3.53 ÎŒm emission features in HD 97048 and Elias 1 to diamondoidsof a few nanometers in size as well as the suggestion that smallerdiamondoid molecules contribute to the 3.47 ÎŒm interstellar absorptionband.